1. Field of the Invention
The invention relates to a power control apparatus and method for transferring electric power from a power source to a work producing load and to a power generation system using such apparatus and method. The invention also relates to a power control apparatus and method for efficiently extracting electric power from a power source and to a power generation system using such apparatus and method.
2. Related Background Art
Since concern for the earth's environment is increasing, a power generation system using wind power generation, a solar battery, or the like which provides a safe and clean energy source is desirable. However, the power output of a solar battery or the like fluctuates extremely depending on various parameters such as insolation amount, atmospheric temperature, operation point voltage, and the like. As a result adjustments are constantly required to ensure that the electrical power supplied by the solar battery to the load produces maximum power transfer. For this purpose, various kinds of apparatuses and methods have conventionally been proposed. For example, in the case of the solar battery, a method whereby meteorological conditions which momently change are used and the operation voltage is determined by using a monitor comprising an auxiliary solar battery or a sensor which is provided separately from a solar battery array and is used to detect the ambient light or a temperature. This method called an indirect method has been disclosed in JP-B-61-2206. On the other hand, a method whereby such an auxiliary sensor is not used is one where an operation point voltage or current of a solar battery array is finely fluctuated and the electric power fluctuation at that time is checked so that an operation voltage can be determined. This method called a direct method has also been disclosed in JP-B-63-57807. Hitherto, a power converter or the like has been controlled by extracting the maximum output from the solar battery by using the methods mentioned above.
The above methods, however, have the following problems.
According to the indirect method, it is necessary to use the monitor to detect an insolation amount or the like. A sensor or solar battery must also be provided separately from the solar battery array to accurately determine the relation between the meteorological conditions which were measured by the monitor or the parameters indicative of the meteorological conditions and the operation point voltage and the power of the solar battery array to be controlled. In order to accurately determine such relation, it is necessary to precisely measure them for a long period of time, which is very troublesome. In addition, the area of a solar battery array is generally equal to or larger than a few square meters, while the monitor is much smaller so that the occurrence of a measurement error or the like is possible. For example, in the case of detecting an insolation amount, if the monitor portion is covered by a shadow due to Some circumstances, in spite of the fact that solar light is being irradiated on the entire solar battery array, a judgment of "dark" is made which prevents the correct control function from being performed.
According to the direct method, since an auxiliary monitor is not used, the maximum power point of the solar battery array can always be tracked. According to this method, however, in order to raise the tracking precision, it is necessary to finely check the optimum operation voltage over a wide range. Generally, the operation point voltage required to obtain the maximum power of the solar battery can fluctuate by as much as 10% depending on the meteorological conditions. For example, in case of the solar battery array whose output voltage is equal to about 200V, the optimum operation voltage exists in a range of about 180 to 220 V. Therefore, in order to obtain the optimum operation voltage, the above range must be finely examined. It is, however, difficult to always momentarily scan the whole range. In JP-B-63-57807 or JP-A-62-85312, an inspected range is narrowed by using a voltage differentiated value of electric power or by introducing a mark indicative of the change of direction of an operation point voltage. According to such a method, when the meteorological conditions fluctuate, the fluctuation of the operation point is gradually traced, so that a fluctuation width must be properly determined in consideration of the relation between the tracking precision and the transient response speed. Because the optimum fluctuation value varies depending on the location of installation, the kind of solar battery, or the like, it is difficult to accurately obtain the optimum fluctuation value. Since the above method uses a differentiated value or the like which is very sharp, there is a problem such that it is largely influenced by a measuring system error or a sudden insolation change which causes an excessive response to occur. On the other hand, according to JP-A-62-42213, a fluctuation is introduced to an output of the solar battery, the voltage and current are sampled, and the point at which the maximum electric power is obtained is directly determined. According to such a method, although stability is high as compared with the above described method, in order to raise the tracking precision, it is necessary to finely examine a wide range and the sampling number must be increased accordingly. Both of the tracking precision and the response speed cannot simultaneously be satisfied.